29: Kent Brooks: Mantle xenoliths and dislocated shoulders

In this episode we hear more from Kent Brooks, emeritus Professor at the Geological Museum in Copenhagen, about the chance discovery of an unusual rock he picked up in East Greenland that led to years of productive research about the nature of the Earth’s mantle far beneath the Earth’s surface.

Transcript

Transcript

29: Kent Brooks: Mantle xenoliths and dislocated shoulders

Based on interviews held on January 9–10, 2020 in Kendal, England

Note: Polar Podcasts are designed to be heard. If you are able, please listen to the audio, which includes emotion and emphasis that is not evident in the transcript.

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Kent 0:01

And I regard this as quite a, quite an exciting bit of work really. But it’s strange how, how it began with a, um, you know a casual, a casual picking up of a rock on a beach.

Julie 0:14

Welcome to Polar Podcasts, where you’ll hear stories from geologists who’ve spent their careers, their lives, exploring and studying the remarkable and remote geology of Greenland . Why did they become fascinated with Greenland? What were the problems and the discoveries that drove them? And what was it like working in these remote places, where few people venture, even now? I’m Julie Hollis.

In this episode we hear more from Kent Brooks, emeritus Professor at the Geological Museum in Copenhagen, about the chance discovery of an unusual rock he picked up in East Greenland that led to years of productive research about the nature of the Earth’s mantle far beneath the Earth’s surface.

Kent 1:01

Well one of the things that er, one of the projects we er, had, which I was quite proud of, it was something that er, that started years ago, when I was in Nordic, Nordic Mining Company, Nordiskemineselskab in er, the 19, 1970. Er we were flying, at that time we were flying up the coast from, from er Kangerlussuaq er, to er look at the various intrusions along the Blosseville coast. Most prominent was the Lilloise Intrusion. And in order to do that we had to lay out fuel depots. But we were just about to go out on this trip when we er, got a radio message that there’d been an accident at, at Wiedemans fjord, that’s by the Lilloise Instrusion. There’d been an accident and we needed to have a medevac. And so our helicopter flew, flew up there. And er, a chap called Peter Brown, who I’ve mentioned before. He was er, my neighbor as a youth er, surprisingly enough. He’d been crossing the Kronborg Glacier, the same glacier that we were er, we were unable to cross on foot. And he’d er, he’d, he’d fallen and his rucksack had swung around somehow and put his, put his arm out of joint. And er, they couldn’t, they couldn’t er, put it back in again. He’d it dislocated it. They couldn’t, they couldn’t repair it.

But we had a doctor with us so er, we could fly him out and the doctor would put it back. He actually did it. He was an Austrian doctor and er, he laid Peter on his back and put a boot under his armpit and then got hold of his hand and gave it a mighty jerk. And there was a sort of cracking noise and er, Peter screamed in agony. But then found that his arm was back in joint again.

Anyhow, we medevaced him out of Lilloise and er, or Wiedemans fjord. And while, while the helicopter was stopped there, I just happened to notice by the skid of the helicopter an interesting looking, looking bit of rock. And so er, I just picked this thing up, shoved it, shoved it in the chopper. And er, later on I, I had it thin sectioned and I had a look at it.

Julie 3:06

A thin section of a rock allows geologists to identify minerals under a microscope.

Kent 3:11

And it turned out to be er, quite unusual. And I, I rapidly identified er, fragments of mantle, mantle material in it. That is I found small crystals of, crystals of er, chromite or chrome spinel rather um, enstatite – orthopyroxene – and er, olivines and then er, small, small, small aggregates of enstatite, olivine, and chrome, chrome, er chrome spinel.

Julie 3:43

These are minerals that are rare in the crust but which make up much of the Earth’s mantle.

Kent 3:49

And er, these were analyzed on the, on the microprobe and we published a little paper in Contributions to Mineralogy and Petrology. Well er, Peter Keleman, who was at the er, Woods Hole Oceanographic Institute was aware of this paper. And when we were er, having an expedition in 1995 under the auspices of Dansk Lithosphere Centre

Julie 4:14

The Danish Lithosphere Centre was a geological research centre based in Copenhagen in the 1990s to early 2000s.

Kent 4:22

Er, he er, he got his eye on this and said we’d go back and look for more of this material. And so I said, “That’s fine.” Peter Keleman, I should say, is er, is an outstanding, outstanding geochemist and petrologist. He now has a, a distinguished chair er, in Columbia University at the Lamont Oceanographic Observatory. At that time he was at the Woods Hole Oceanographic Observatory in Cape Cod.

Anyhow we er, I was not at all clear whether, whether we could find any more of this stuff because of course I didn’t know. I just picked up the one rock. It might just have been the one rock that was there and would we be able to find the rest of it? Well we flew up there and er, as far as I could remember, I mean, what had gone past, 18, 19, more than 20 years had passed. And er, I couldn’t quite remember where we’d landed. But er, we landed where I thought we’d been. And we got out. And lo and beyond we did, we found, found quite a lot more. We spent some days there and we loaded up quite a few box, boxes of rocks with this stuff.

And er, this was another payload problem because the, the helicopter was load, loaded so heavily with these rocks that er, it could, it could scarcely get off the ground. With helicopters, they have a torque monitor. And if the torque, if the torque, if this goes over, much over a hundred percent you, you have a matter of seconds before the engine burns out. So you mustn’t, you mustn’t sort of put far too much torque on the motor like that.

In this case, we had er, we just managed to get airborne. And of course once you get airborne it’s alright because the er, the weight is decreasing all the time as you burn the fuel. It’s a just a question of getting off the ground. If you can’t get off the ground that’s er, the end of it. And you have to take something out.

Anyhow, it, we, we’d collected a whole lot of stuff here. And er, Stefan worked on this mainly.

Julie 6:22

Stefan Bernstein, a former PhD student of Kent’s and now Head of the Department of Petrology and Economic Geology at the Geological Survey of Denmark and Greenland.

Kent 6:32

And it turned out that the rocks were er, these, these nodules of er, mantle rocks, they were um, extremely depleted. That is to say that the er, the er easily fusible elements

Julie 6:47

Those that can be melted easily.

Kent 6:49

that er, when a rock undergoes partial melting some, some elements melt out before the others. And you’re left with a, with a, an infusible residue. These were, these were rocks which had an enormous amount of material melted out of them. And there was only this infertile residue left over. That is we could, you could see that because the composition of the olivine, which is er very magnesium-rich.

Anyhow, Stefan found out that er, these, these rocks could be the er, the residual material from the melting of a type of rock called komatiite. Komatiites are only found in the Precambrian.

Julie 7:30

The geological time period from the earliest history of the Earth until about 540 million years ago.

Kent 7:38

They’re er, very magnesium-rich lavas. Er, much more magnesium-rich than, than normal basalts. And er, they’re not known from, from younger rocks than the Precambrian. And the reason for this is thought to be that during the early stages of Earth’s history, the er, geothermal gradient was much higher than it is today, that is the Earth was much hotter. And so er, things, things, things melted at higher temperatures. The volcanic rocks were produced at higher temperatures.

Anyhow, it appeared that these, these minerals and nodules in the rocks of Wiedemans fjord, they were er, residues from the melting of, the melting of rocks in the, in the early Precambrian.

Of course, we didn’t really know the age of it. We didn’t get any, any grasp of the age of it. But later on we were able to show that in fact these things were, did date from the Precambrian, by using the so-called rhenium-osmium model ages. And that was work that Karen Hanghøj did

Julie 8:35

Karen Hanghøj is a former PhD student of Kent’s and is now director of the British Geological Survey, the first woman to hold the role in the Survey’s 185 year history.

Kent 8:46

when she er, spent her time at Woods Hole. Er she determined the rhenium, rhenium and osmium isotopic composition of the rocks and was able to make a, make a, a guess as to what kind of time, what the time was when the melting had taken place.

And I regard this as quite a, quite an exciting bit of work really. And it’s strange how, how it began with a, um, you know a casual, a casual picking up of a rock on a beach.

We extended the work subsequently by going to, going to, going to West Greenland. And Stefan and I spent a summer on Ubekendt Island er, collecting similar, similar rocks from West Greenland. Most, most er, most notably, some of the rocks on the west coast of Ubekendt Island that we called er, Dead Dog’s Gulch because it had a dead dog in it, which was rather uncomfortable because er, because the, the best exposure we found there was only a few yards from the carcass of this smelly dog. And quite why, why, why the dog had chosen to die there I’ve no idea but it was, it was very inconvenient it happened to be there. It could have chosen from the whole coastline. Anyhow, this dyke, this dyke was absolutely packed full of nodules. It was spectacular, very spectacular rock was the Dead Dog’s Dyke.

Julie 10:07

The nodules Kent’s referring to are pieces of the Earth’s mantle, which are rarely found at the Earth’s surface.

Kent 10:12

And er, it yielded very similar results to those we got from Wiedemans fjord. And I think in all we got er, four or five papers out of this project.

Julie 10:23

I’m Julie Hollis and you’ve been listening to Polar Podcasts.

Julie 10:35

In the next episode, we hear more from emeritus senior scientist Bjørn Thomassen about severe storms while exploring for gold in northwest Greenland.